Kitchen waste is the food waste and residues of families, dining rooms and food and beverage industry, which is the important component of urban domestic waste. Kitchen waste has a high water content and is rich in nutrient substances, wherein the organic content accounts for over 95% of the dry matter, which is rich in starches, saccharides, proteins and fats, etc. Kitchen waste also contains vitamins and microelements such as nitrogen, phosphorus, sulfur, potassium, calcium and magnesium. The nutrient elements are complete and can be reused by organisms. However, just because kitchen waste has a high water content and is rich in nutrient substances, under normal temperature conditions, microorganisms will use various organics and inorganic salts to rapidly breed and metabolize, so that the kitchen waste will rot and smell, polluting the environment and being very difficult to deal with. The amount of kitchen waste is huge, and in China, the annual amount of kitchen waste is over 60 million tons. However, at present, most of kitchen waste is used to feed pigs, landfills, or even be made to “waste oils”, which cause serious environmental pollution. Only a small part is used reasonably, such as incineration, composting and being made to marsh gas, however, the economic benefits are low. Therefore, finding a way to fully utilize the kitchen waste is an important issue in the treatment of kitchen waste.
Since kitchen waste is rich in organics such as starches, saccharides and proteins, it is an ideal renewable material. Using kitchen waste as the raw material for the production of non-food fuel ethanol is a promising development direction. On the one hand, it can solve the problem of resource utilization of kitchen waste, which can not only turn waste into treasure, and improve economic efficiency, but also solve environmental pollution problems, with enormous social benefits. On the other hand, using kitchen waste as the raw material can solve the problem that the raw material of fuel ethanol production in China right now is mainly based on grain starch, which pushes up the price of food and causes food crisis.
Candida utilis, also known as round protein-producing yeast or edible round yeast, is a safe yeast (GRAS, Generally Recognized as Safe) approved by US Food and Drug Administration (FDA), and also a “fungus which can be used for health food” approved by China Food and Drug Administration. Candida utilis is an important industrial yeast, and is used for producing a variety of high-value-added biological agents, such as glutathione, RNA, amylase, L-lactic acid and carotenoid. Similar to Saccharomyces cerevisiae, Candida utilis can produce ethanol by glucose fermentation and has a comparable sugar alcohol conversion rate to that of Saccharomyces cerevisiae. Furthermore, compared with Saccharomyces cerevisiae, Candida utilis has the following advantages: (1) Candida utilis is Crabtree-negative, and when growing in a glucose-rich medium, its respiration is not inhibited and ethanol is not produced; and it can grow rapidly under a strict aerobic condition; (2) Candida utilis has a high fermentation density, and under an effective continuous culture condition, it can achieve a high density culture, wherein the dry weight of cells can reach 92 g/L, which is beneficial to produce the desired products efficiently; (3) Candida utilis can use cheap molasses and wood hydrolysate as the nutrient for growth, saving production costs; (4) Candida utilis can use pentoses and hexoses as carbon sources at the same time; (5) Candida utilis is adaptive to various carbon sources and nitrogen sources (comprising urea and nitric acid), and its protein content and Vitamin B content are higher than those of Saccharomyces cerevisiae; (6) its secretory proteome contains no protease, which is conducive to the expression of heterologous proteins as a host strain; and (7) Candida utilis is an edible and safe single-cell protein whose products and the fungal cells per se can be directly used as additives in industries of food, pharmaceuticals, and cosmetics without complex separation and purification procedures, thus saving time and labors.
However, Candida utilis lacks enzymes for effectively degrading starch into glucose, and degrading proteins into polypeptides and amino acids, so it cannot directly use starch and proteins in kitchen waste as the carbon sources and nitrogen sources to produce ethanol by fermentation. Therefore, for using kitchen waste as the raw material to produce ethanol by fermentation, it is necessary to introduce the genes of enzymes which are capable of degrading starch and proteins into Candida utilis by genetic engineering, to compensate for the defect in its ability of degrading kitchen waste. Therefore, the recombinant Candida utilis can degrade starch and proteins in kitchen waste into available carbon sources and nitrogen sources using the amylase and protease synthesized itself, thereby realizing the industrial purpose of using the fermentation of kitchen waste for producing ethanol.